Many techniques have been developed in the past to provide electrical bonding of the eyelets and the heads of the pins.
A first conventional technique consists in applying a solder ring around the head of the pin and in then heating the module in a furnace. A description of this technique may be found in an article entitled "solder Ring Application Tool", by R. F. Buczac et al in IBM Technical Disclosure Bulletin, Vol. 12, No. 1, June 1969, pages 15-16.
In practice, this technique implies the use of cylindrical (i.e. headless) pins and as a result complicates the handling of substrates provided with such pins. Also, any slight displacement or faulty positioning of the solder ring may result in a defective bond or even in a short circuit between the heads of two pins.
It has also been proposed to effect the bond in an electrolytic bath as described in the article entitled "Pin Bonding Technique", by F. C. Campagna et al, in IBM Technical Disclosure Bulletin, Vol. 14, No. 11, April 1972, pages 3246 and 3247. This technique, in addition to being fairly complex, is highly contaminating, which can be detrimental to the reliability of the encapsulated module.
The substrates can also be tinned, using the well-known dipping technique. However, this technique is very expensive in terms of raw materials (flux, solder, perchlorethylene) and manpower (tinning and visual sorting).
Another conventional technique consists in tinning the substrates by means of a solder paste that is dispensed during positioning of the chip. This technique has two major disadvantages: the step of dispensing the paste onto the heads of the pins is difficult to perform, and solder residues, which are frequently present, may cause short circuits between the conductors.
A similar technique is described in an article entitled "Solder Creme Dispensing Nozzle", by G. Sabol and J. Siwy, in IBM Technical Disclosure Bulletin, Vol. 14, No. 7, December 1971, Page 2090.
All of the above techniques require that the heads of the connecting pins be bonded to the eyelets of the conductors before the solder balls on the semiconductor chips are bonded to the conductive fingers on the ceramic substrate. In such cases, some solder frequently happens to come in contact with said fingers, thereby making the chip more difficult to align. One of the solutions that have been proposed consists in performing an additional brushing step to remove such unwanted solder.